Local oxidation of silicon (LOCOS) is a method of forming field oxide regions on semiconductive material wafers. The field oxide regions can be utilized to electrically separate adjacent electrical devices which are formed over the semiconductive material wafer subsequent to the formation of the field oxide regions. A LOCOS process is described with reference to FIGS. 1-5.
Referring to FIG. 1, a semiconductive material wafer fragment 10 is illustrated at a preliminary step of a prior art LOCOS process. Wafer fragment 10 comprises a semiconductive material substrate 12 having a pad oxide layer 14 and a silicon nitride layer 16 formed thereover. Pad oxide layer 14 can comprise, for example, silicon dioxide, and is typically from about 20 nanometers to about 60 nanometers thick. Silicon nitride layer 16 is typically from about 100 nanometers to about 200 nanometers thick. Substrate 12 can comprise, for example, lightly doped monocrystalline silicon. To aid in interpretation of the claims that follow, the term "semiconductive substrate" or "semiconductor substrate" is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term "substrate" refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.
A patterned masking layer 18 is provided over silicon nitride layer 16. Patterned masking layer 18 can comprise, for example, photoresist patterned by a photolithographic process. Patterned masking layer 18 covers some portions (labeled as 20) of silicon nitride layer 16, and leaves other portions (labeled as 22) uncovered.
Referring to FIG. 2, wafer fragment 10 is subjected to etching conditions which remove uncovered portions 22 (FIG. 1) of silicon nitride material 16 to form openings 26. The etching also extends through pad oxide layer 14 and partially into silicon layer 12. Openings 26 can extend to, for example, about 500 .ANG. into substrate 12.
The etching of openings 26 forms covered portions 20 of pad oxide 14 and silicon nitride 16 into masking blocks 30. Such masking blocks have opposing sidewall edges 32 and 34 (which are labeled only for the center masking block shown in FIG. 2). Also, the etching of openings 26 into substrate 12 forms pedestals 36 of the substrate material. Pedestals 36 have opposing sidewall surfaces coextensive with sidewall surfaces 32 and 34 of masking blocks 30.
Referring to FIG. 3, masking layer 18 (FIG. 2) is removed and silicon nitride projections 40, 42, 44 and 46 are formed along the sidewall edges of masking blocks 30 and pillars 36. Silicon nitride projections 40, 42, 44 and 46 can be formed to a thickness "T" of, for example, from about 100 .ANG. to about 200 .ANG., and can be formed by depositing and anisotropically etching a layer of silicon nitride.
Referring to FIG. 4, wafer fragment 10 is subjected to oxidizing conditions to form field oxide regions 50. The oxidizing conditions can comprise, for example, wet oxidation conducted at temperatures of about 1,000.degree. C. for a time of from about 2 hours to about 4 hours. The oxidation grows silicon dioxide from portions of substrate 12 between masking blocks 30. The growing silicon dioxide extends to under nitride projections 40, 42, 44 and 46 to from slight birds beak projections extending under silicon nitride layer 16 of masking blocks 30. Projections 40, 42, 44 and 46 limit an extent to which the oxide grows to under nitride layer 16 of masking blocks 30, and accordingly limits an amount of bird's beak formation.
Referring to FIG. 5, nitride layers 16, 40, 42, 44 and 46 are removed to leave field oxide 50 over substrate 12. Field oxide 50 has dips 52 formed therein where nitride projections 40, 42, 44 and 46 (FIG. 4) had been. Active area regions 31 are defined as regions between field oxide regions 50.
Pad oxide 14 remains over active area regions 31. In subsequent processing (not shown), pad oxide 14 can be stripped and replaced with another oxide layer. Subsequently, semiconductor devices, such as, for example, transistors can be formed between field oxide regions 50. Such devices will then be electrically separated from one another by field oxide regions 50.
Several difficulties occur in the processing described above with reference to FIGS. 1-5. Specifically, if nitride projections 40, 42, 44 and 46 are too thin, there will be excessive bird's beak encroachment under nitride layer 16. On the other hand, if nitride projections 40, 42, 44 and 46 are too thick, dips 52 will be excessively large, and will lead to sub-threshold kinks and other problems with circuitry ultimately formed over active area regions 31. It would therefore be desirable to develop alternative methods of LOCOS processing.